It is proposed to study the determinants coded for by genes of the major histocompatibility complex in man, HLA, primarily through the use of primed lymphocyte testing (PLT) reagents. It would appear that the different types of determinants that can be detected and related to the HLA-D region, including HLA-D determinants detected with homozygous typing cells and HLA-DR determinants detected serologically, can both be detected by PLT cells. It is not clear whether the determinants detected by PLT cells are identical with those detected with HTCs and serology or are related to them in the population. Since normal PLT cells, even when highly discriminatory, still give many intermediate responses that are difficult to interpret, suggesting complexity of determinants associated with even a single HLA-D haplotype, we are investigating the use of cloned PLT reagents for the study of this problem. PLT cells are cloned by utilizing day 4 blasts from a priming MLC which are then seeded at limiting dilution values in wells of microplates and grown in the presence of appropriate feeder cells and T cell growth factor. The attempt will be made to grow up PLT-reactive clones that recognize different determinants in large numbers so that genetic studies can be done in both families and at the population level.